Assuming we are going to get Boateng, then I'd agree with most on here that we should make centre midfield and left back the priorities. Sneijder would obviously be great, otherwise Gerrard, perhaps Van de Vaart. For left back I'm not sure - Lahm obviously woudn't come, saw someone put Clichy which might be a decent shout. Otherwise perhaps even Kieron Gibbs - surely he must be looking to make a breakthrough as a first-team regular this year, and he's not going to do it with Clichy there.
Given how many games we are going to play next season (assuming a decent run in the Europa!), I think it's going to be important to add more options, not only in terms of like-for-like replacements, but also in terms of different systems/formations. So, for example, if we sign a new left back and top central midfielder we might play:
--------------Given/Hart-----------
Boateng---Kompany----Lescott-----New
A. Johnson----New-----Barry-------Bellamy
--------------Tevez
---------------------Adebayor
Or a front six of:
----------------De Jong
-----------New---------Barry
A. Johnson-------------------Bellamy
-----------------Tevez
Or perhaps even:
A. Johnson---De Jong---Barry---Bellamy
--------------New (e.g. if Gerrard)
-----------------Tevez
If we were to make any other signings, I would go for a striker with genuine pace - not sure who, but someone to give us a different option, and who could play up front either with Tevez tucked in behind him, or alongside Adebayor. Beyond that, I think any further signings will depend entirely on the attitude of certain players over the summer and their willingness to knuckle down - if Richards, Wright-Phillips, Ireland, even Robinho come back in willing to work and looking like getting back to somewhere close to their best, then I honestly don't think we would need anymore signings to be genuinely challenging on all fronts (If they don't, then obviously we will need to look for other signings). Assuming the first of the two teams above, our squad options would be:
GK: Given/Hart
RB: Richards/Zabaleta
LB: Bridge (?)
CB: Toure, Onuoha (+ Boateng)
RW: Wright-Phillips (Weiss?)
CM: De Jong/Ireland/M. Johnson (?)
LW: Robinho (+ A. Johnson)
ST: New (+ Bellamy)
Certainly enough for a top 4 challenge in my opinion.